Advanced heart failure is a major global health problem resulting in many thousands of deaths each year and those with the disease endure a very poor quality of life. The treatment options for advanced heart failure, for example drug therapy and cardiac resynchronization (pacemakers), have generally proved unsuccessful and the only option remaining for the patients is heart transplantation. Unfortunately, the number of donor hearts available only meets a fraction of the demand, leaving many people untreated.
Ventricular Assist Devices (VAD) have been gaining increased acceptance over the last decade as an alternative therapy to heart transplantation. The use of VADs has shown that, in most cases, once the device has been implanted, the disease progression is halted, the symptoms of heart failure are relieved, and the patient regains a good quality of life.
VADs can be considered as a viable alternative to treat heart failure and offer hope to the many thousands of heart failure patients for whom a donor heart will not be available.
In general terms, it is known to provide a cardiac pump, such as a VAD, that is suitable for implantation into a ventricle of a human heart. The most common type of these implantable pumps is a miniaturised rotary pump, due to their small size and mechanical simplicity/reliability. Such known devices have two primary components: a cardiac pump housing, which defines a cardiac pump inlet and a cardiac pump outlet; and a cardiac pump rotor, which is housed within the cardiac pump housing, and which is configured to impart energy to the fluid.
A requirement for the cardiac pump, therefore, is a bearing system that rotatably supports the cardiac pump rotor within the cardiac pump housing. Bearings systems for cardiac pumps, and generally all rotating machines such as pumps and motors, ideally achieve the fundamental function of permitting rotation of a the rotor, whilst providing sufficient constraint to the rotor in all other degrees of freedom, i.e. the bearing system must support the rotor axially, radially and in pitch/yaw.
Desirable functions of bearing systems generally may include low rates of wear and low noise and vibration, and the case of blood pumps, elimination of features that trap blood, or introduce shear stress or heat in the blood.
In known devices, the cardiac pump rotor may be rotatably supported within the housing using one of a number of different types of bearing systems. In general, there are three types of bearing systems that are utilised in cardiac pumps.
Some cardiac pumps use blood-immersed contact bearings, for example a pair of plain bearings, to rigidly support the rotor within the housing. However, for such plain bearing systems it may be difficult to ensure that the rotor is perfectly entrapped within the contact bearings. Moreover, blood-immersed contact bearings of the prior art may be susceptible to proteinacious and other biological deposition in the bearings, and also the region proximate to and supporting structures around the contact bearings.
Other cardiac pumps use non-contact hydrodynamic bearing systems, in which the rotor is supported on a thin film of blood. In order to produce the required levels hydrodynamic lift, hydrodynamic bearing systems require small running clearances. As a consequence, blood that passes through those small running clearances may be subjected to high levels of shear stress, which may have a detrimental effect on the cellular components of the blood, for example by causing haemolysis or platelet activation which may further lead to thrombosis.
Cardiac pumps may also employ non-contact magnetic bearing systems, in which the running clearances between the rotor and the housing may be designed such that large gaps can exist in the bearing and therefore shear-related blood damage in the bearing is reduced. However, due to the limitations resulting from Earnshaw's theorem, a passive magnetic bearing system requires another manner of support in at least one degree-of-freedom, for example active magnetic control, which may significantly increase the size and complexity of the design, and/or hydrodynamic suspension, which may increase the requirements with regard to manufacturing tolerances or introduce blood damage.